1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular to an electrophotographic image forming apparatus for expressing gradation with print area ratio.
2. Description of Prior Art
In an image forming apparatus, it is required to form an image with reproducible colors and densities. Then, before the process of forming an image according to image data is started, a standard toner image is formed in predetermined standard conditions in a non-image area on a photoconductor which is not related to the image forming, and the density of the standard toner image is detected with a density sensor. Then, image forming conditions such as the developing bias voltage, the surface potential of the photoconductor, the optical intensity of exposure are controlled according to the detected density.
However, the toner density to be detected is affected by the charging characteristic of the photoconductor, the development bias characteristic or the like. Especially for a standard toner image consisting of a plurality of dots, the size or diameter of the dots is changed, and this changes the toner density. If the humidity or temperature is changed, the amount of charges on toners per unit weight is affected, and this changes the density of the standard toner image. If the image forming conditions are set according to the density changed by the above-mentioned factors, a desired image may not be obtained.
Further, even if the density of the standard toner image is detected correctly, when the transfer efficiency is changed, a desired image may not be obtained. Therefore, transfer efficiency has to be taken into account in the image stabilization.